This invention is in the field of electrical and electrochemical devices for storing or harnessing energy. This invention relates generally to management of such devices to reduce the severity of consequences of abnormalities occurring or existing in the devices. Batteries are a prominent example of a type device to which this invention relates.
As energy device technology development has progressed, the use of batteries, particularly rechargeable batteries, as a power source has increased substantially. Batteries are used as power sources for a wide array of devices including relatively low-power devices, such as consumer electronics devices, and higher-power devices, such as electric cars. Lithium ion energy device are the most widely used form of rechargeable battery. An Achilles heel of lithium ion energy devices is the risk of an electrical short developing inside a lithium ion energy device cell and the consequences associated therewith. An electrical short may cause rapid heating of the energy device cell. In the matter of seconds, the local temperature at the location of the short may rise to temperatures sufficient to set the energy device on fire. This is particularly worrisome in the case of high-capacity lithium ion energy device systems, such as those used in electric cars.
Current energy device management techniques for detecting state of health and state of charge of the energy device include the EIS method, OCV delay method, and entropy and enthalpy methods.
EIS method measures the resistance of a cell or an electrode, which is a function of the electrode materials chemistry, size of particles and geometry. Knowing the resistance of the cell or electrode helps with characterization of the cell or electrode material, such as its kinetics, which can be used to estimate the performance, as well as state parameters of the electrode or the cell, including the state of charge and state of health of batteries. This method requires generating AC signals with a wide range of frequencies, typically from 100 kHz to 100 mHz.
Open Circuit Voltage (OCV) may be measured easily at rest, however measuring OCV during usage of the energy device (cycling) is not a simple operation. Part of the difficulty is due to the kinetics of the reactions that result in delays of the reading voltage getting stabilized. Another difficulty in estimating the internal parameters of the energy device via OCV is that the relationship between OCV and the energy device internal parameters such as state of charge and state of health may not be easy to use, especially in Li-ion cells with LiFePO4 cathode the voltage variations due to state of charge changes are not significant and thus estimating state of charge based on voltage values is not practical. In the OCV delay method, the speed at which the OCV value stabilizes may be used to estimate the internal states of the energy device such as state of charge and state of health. This method, however, requires precise measurement of the voltage of the energy device and thus requires costly hardware for achieving such measurements.
Entropy and enthalpy measurement methods to estimate the state of charge and state of health of the energy device also detriment from requirement of costly hardware to precisely measure the energy device temperature and OCV.
The above discussed measurement methods therefore disadvantageously require costly hardware and further require cycling of the energy device. Therefore, measurement time is increased. These methods typically require precise and complex instruments and can only be applied to single cells, which limit them to research applications. Sensors, such as thermal and stress-strain sensors, are used in critical applications, such as for vehicle and aerospace applications, however these sensors only detect the secondary outcome of battery problems and only after the damage has progressed extensively. Thus, these sensors may be useful only to detect a catastrophic failure.